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ArticleThe role of magmatism in the thinning and breakup of the South China Sea continental margin: Special Topic: the South China Sea Ocean Drilling(Oxford University Press, 2019-08-13) Sun, Zhen ; Lin, Jian ; Qiu, Ning ; Jian, Zhimin ; Wang, PinXian ; Pang, Xiong ; Zheng, Jinyun ; Zhu, BenduoMagmatism plays a key role in the process of continental margin breakup and ocean formation. Even in the extremely magma-poor Iberia and Newfoundland margin, studies of field outcrops have shown that syn-rift magmatism had participated in rifting from a very early stage and contributed directly to the rifting process. The final transition from exhumed continental mantle to the ocean formation is also triggered by the accumulation and eruption of magma [1]. Therefore, Atlantic-type passive continental margins are classified into two end-members: magma-poor (non-volcanic) and magma-rich (volcanic). The differences between them lie in whether a large amount of intrusive and extrusive magmatism from the mantle plume/hotspot is involved in the syn-rift and breakup stages. A magma-rich margin [2] should include the following characteristics: (i) a high-velocity lower crust (HVLC) caused by syn-rift mafic magma underplating; (ii) continental crust intruded by abundant sills and dikes; (iii) a large volume of seaward-dipping reflectors (SDRs) caused by flood basalt eruption or tuffs. All other margins are classified as magma-poor margins.
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ArticleThe latest spreading periods of the south china sea: new constraints from macrostructure analysis of IODP expedition 349 cores and geophysical data(American Geophysical Union, 2019-08-29) Sun, Zhen ; Ding, Weiwei ; Zhao, Xixi ; Qiu, Ning ; Lin, Jian ; Li, Chun‐FengMacrostructures preserved in deformed rocks are essential for the understanding of their evolution, especially when the deformation is weak and hard to discriminate in regional scale or purely through geophysical data. In order to resolve the inconsistency between NS trending fracture zones and NE oriented spreading fabrics of the South China Sea during the latest spreading stage, we analyzed macrostructures identifiable from the basalt and consolidated sediment samples of the Integrated Ocean Drilling Program (IODP) Sites U1431 and U1433. These two sites are close to the East and Southwest relict spreading ridges and provide critical information on the latest spreading stages. The structures in the basalt of both sites suggest two dominant orientations of NS and NE. At U1431, sediments show mainly WNW trending slickensides, different from that of basalt. At U1433, no structures were found in postspreading sediment. Thus, NE and NS trending structures in basalt are most possibly formed by seafloor spreading. Crosscutting relationship suggests that NE trending structures formed first, followed by NS and finally WNW trending structures. These observations are consistent with geophysical features. Magnetic anomalies and ocean bottom seismometer velocity suggest that the latest relict ridge of the East Subbasin coincides with the EW trending seamount chain. Located between the relict ridges of East and Southwest Subbasins, NS trending Zhongnan‐Liyue Fracture Zone had acted as the latest transform fault. Based on the above evidences, we proposed that the South China Sea may have experienced a short period of NS oriented spreading after earlier SE spreading. These results resolve the previous inconsistencies.
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ArticleAges and magnetic structures of the South China Sea constrained by deep tow magnetic surveys and IODP Expedition 349(John Wiley & Sons, 2014-12-27) Li, Chun-Feng ; Xu, Xing ; Lin, Jian ; Sun, Zhen ; Zhu, Jian ; Yao, Yongjian ; Zhao, Xixi ; Liu, Qingsong ; Kulhanek, Denise K. ; Wang, Jian ; Song, Taoran ; Zhao, Junfeng ; Qiu, Ning ; Guan, Yongxian ; Zhou, Zhiyuan ; Williams, Trevor ; Bao, Rui ; Briais, Anne ; Brown, Elizabeth A. ; Chen, Yifeng ; Clift, Peter D. ; Colwell, Frederick S. ; Dadd, Kelsie A. ; Ding, Weiwei ; Almeida, Ivan Hernandez ; Huang, Xiao-Long ; Hyun, Sangmin ; Jiang, Tao ; Koppers, Anthony A. P. ; Li, Qianyu ; Liu, Chuanlian ; Liu, Zhifei ; Nagai, Renata H. ; Peleo-Alampay, Alyssa ; Su, Xin ; Tejada, Maria Luisa G. ; Trinh, Hai Son ; Yeh, Yi-Ching ; Zhang, Chuanlun ; Zhang, Fan ; Zhang, Guo-LiangCombined analyses of deep tow magnetic anomalies and International Ocean Discovery Program Expedition 349 cores show that initial seafloor spreading started around 33 Ma in the northeastern South China Sea (SCS), but varied slightly by 1–2 Myr along the northern continent-ocean boundary (COB). A southward ridge jump of ∼20 km occurred around 23.6 Ma in the East Subbasin; this timing also slightly varied along the ridge and was coeval to the onset of seafloor spreading in the Southwest Subbasin, which propagated for about 400 km southwestward from ∼23.6 to ∼21.5 Ma. The terminal age of seafloor spreading is ∼15 Ma in the East Subbasin and ∼16 Ma in the Southwest Subbasin. The full spreading rate in the East Subbasin varied largely from ∼20 to ∼80 km/Myr, but mostly decreased with time except for the period between ∼26.0 Ma and the ridge jump (∼23.6 Ma), within which the rate was the fastest at ∼70 km/Myr on average. The spreading rates are not correlated, in most cases, to magnetic anomaly amplitudes that reflect basement magnetization contrasts. Shipboard magnetic measurements reveal at least one magnetic reversal in the top 100 m of basaltic layers, in addition to large vertical intensity variations. These complexities are caused by late-stage lava flows that are magnetized in a different polarity from the primary basaltic layer emplaced during the main phase of crustal accretion. Deep tow magnetic modeling also reveals this smearing in basement magnetizations by incorporating a contamination coefficient of 0.5, which partly alleviates the problem of assuming a magnetic blocking model of constant thickness and uniform magnetization. The primary contribution to magnetic anomalies of the SCS is not in the top 100 m of the igneous basement.